The present invention relates to a process for the manufacture of shaped products by the compression of a wet mixture of calcium sulphate dihydrate, cement, water and particulate inorganic materials such as sand.
A process for the manufacture of shaped products by compression of calcium sulphate dihydrate (CaSO.sub.4.2H.sub.2 O) is disclosed in the published Netherlands patent application No. 7410706. The calcium sulphate may be in the form of chemical gypsum obtained from the chemical industry, for example as a by-product in the preparation of phosphoric acid or natural gypsum. It has also been found that shaped articles can be made by compressing calcium sulphate dihydrate together with known additives such as calcined gypsum (CaSO.sub.4.1/2H.sub.2 O), anhydrite (CaSO.sub.4) and lime, or other binding materials. However, until now the manufacture of shaped products by compression of such mixtures has not yielded completely satisfactory results. One or more of the following disadvantages have been experienced:
(1) The process has been uneconomical because very high pressures, in the order of 400 kg/cm.sup.2, must be used to form a shaped product having only a crushing strength of around 150 kg/cm.sup.2, which is generally required to form "bricks" from calcium sulphate dihydrate for load-bearing structures. As used herein, the term crushing strength means the pressure in kg/cm.sup.2 on two opposed sides of the shaped product at which fracture of the shaped product occurs. The presses normally used for forming shaped products do not produce this high a pressure, and presses which are capable of producing such high pressures are not economical to use in a process for the manufacture of shaped products from calcium sulphate dihydrate.
(2) The cost of manufacturing a shaped product from calcium sulphate dihydrate is a function of the cost of the components of the calcium sulphate dihydrate mixture to be compressed, and the cost of the compression itself. Previously the optimum relationship between the components of the mixture and the pressure to be applied to obtain a desired crushing strength has been unknown.
(3) It has not been possible to economically utilize the wet gypsum from the chemical industry, such as from the preparation of phosphoric acid, unless thermal drying is applied, or large quantities of cement, dry calcium sulphate hemihydrate or dry anhydrite and lime are added. It is apparent that these measures are costly, and tend to make the use of wet gypsum uneconomical.
(4) In most instances, such mixtures were difficult to process in a compressing process step.